Polybenzimidazoles, and particularly aromatic polybenzimidazoles, are characterized by a high degree of thermal stability. They may be shaped to form fibers, films and other articles of wide utility which show great resistance to degradation by heat, hydrolytic and oxidizing media.
It has been found, however, that such shaped polybenzimidazole articles darken rapidly, for example, in a few hours, from a relatively light gold color to a dark, coffee or chocolate brown. Although it is not understood, it is believed that the darkening is due to some type of photodegradability of the polymer in which the free amine end groups present in the polymer are affected. For example, it has been postulated that hydrogen peroxide may be formed from the polybenzimidazole polymer under certain conditions, including the presence of air and moisture, which would tend to react with the polymer chain. In addition, it has been suggested that the repeating units of the polymer backbone may be oxidized to a colored quinoid structure.
Regardless, however, the problem is known. A number of solutions have been attempted to impart color stability to the polybenzimidazole materials without success. For example, commercial antioxidants and ultra-violet light absorbers have been added to polybenzimidazole films and other articles. In addition, a number of treatments to polybenzimidazole yarns, films and other shaped articles such as scouring with nitrous acid and other oxidizing or reducing materials, have been attempted. In addition, a chrome treatment of the material has been attempted before dyeing. While some of these treatments have been found to lighten the ground color of the polybenzimidazole material, the material still darkens upon exposure to light.
Commonly assigned U.S. Pat. No. 3,836,500 to Gordon W. Calundann and George R. Ferment discloses that the color stability of polybenzimidazole articles may be improved through the incorporation of certain dissolved stabilizing reagents in a solution of the polymer prior to article formation. Reagents there utilized are: (1) an aliphatic carboxylic acid anhydride having 1 to about 6 carbon atoms, (2) aliphatic carboxylic acid halides having 1 to about 6 carbon atoms, (3) tosyl chloride, and (4) 2,4-dinitrofluorobenzene.
Commonly assigned U.S. Ser. No. 536,807, filed Dec. 27, 1974 (now U.S. Pat. No. 3,942,950), of Edward J. Powers and Walter P. Hassinger, entitled "Improved Process for the Dyeing of Polybenzimidazole Fibers with Anionic Dyestuffs" discloses a technique whereby polybenzimidazole fibers may be successfully dyed while accomplishing complete and uniform dye penetration. The ability to produce satisfactorily dyed polybenzimidazole articles which resist gradual darkening upon exposure to light over extended periods of time is considered to be of prime importance if such articles are to maintain the desired aesthetically pleasing shades in combination with their other highly desirable utilitarian characteristics.
It is an object of the present invention to provide a process wherein the usual color darkening of a polybenzimidazole solid material upon exposure to light is diminished.
It is an object of the present invention to provide a process wherein the ground color inherently exhibited by polybenzimidazole polymer may be stabilized and rendered more constant during use.
It is an object of the present invention to improve the lightfastness of solid polybenzimidazole polymer.
It is an object of the present invention to provide a polybenzimidazole fibrous material exhibiting an enhanced resistance to undergo the usual darkening upon exposure to light.
It is another object of the present invention to provide a polybenzimidazole fibrous material having improved color stabilization without adversely influencing its desirable non-burning characteristics.
It is a further object of the present invention to provide a solution of a polybenzimidazole polymer which is capable of forming polybenzimidazole shaped articles having an enhanced resistance to darkening upon exposure to light.
These and other objects, as well as the scope, nature, and utilization of the present invention will be apparent to those skilled in the art from the following description and appended claims.